# Power detection technology / Power detection

## Detection Technology

### Selection of current value of DC resistance tester

How to choose the current value of the DC resistance tester:

The DC resistance tester is a high-precision replacement product that replaces the DC single and double-arm bridges. It adopts advanced switching power supply technology and liquid crystal display measurement results. It overcomes the shortcomings of other similar products that the LED display value is inconvenient to read under the sun. With automatic arc suppression function.

1. Formula One

When the AC no-load current at the rated voltage passes through the transformer windings, there will be a rated magnetic flux density Bn in the iron core, and the magnetic permeability of the iron core will be relatively low. When measuring DC resistance, it is necessary to make the magnetic density in the iron core greater than Bn, so that the iron core conductivity becomes lower, so as to reduce the time constant of the circuit and the back electromotive force dLi/dt and shorten the stabilization time. Therefore, the DC current when measuring DC resistance should be at least:

I = kˇĚ 2i0In+ 100

Where k: constant> 1 i0: AC rated frequency, percentage of no-load current under rated voltage (%) In: rated current of the winding under test (A), where the constant ˇĚ2 is the equivalent of DC current to AC current Amplitude. When the factor k is larger than 1, the magnetic density in the iron core> Bn, which reduces the magnetic permeability ¦Ě of the iron core during DC resistance measurement. 2. Formula 2

When the transformer winding is star-shaped (Y) connected, the line current is equal to the phase current. From the above formula, the current that should be applied when measuring the DC resistance is:

IY=1.41 ki0In: 100

3. Formula three

When the transformer winding is triangular (D) connection, the no-load current line current of the converter is ˇĚ3 times the phase current, and the DC current when measuring the DC resistance is based on the ratio of the total current] and 1/3 and 2/3 distributed. So the current should be added to measure the DC resistance:

ID = 1.41x3/2+1/ˇĚ3 ki0Inx100 = 1.22 ki0InˇÂ 100

When k is 3~10, that is, when the excitation ampere-turn when measuring DC resistance is 3~10 times of the no-load current ampere-turn, the magnetic density in the core can be greater than Bn and close to saturation, that is, the DC when measuring DC resistance The current is equal to 2%-10% of the rated current. If the DC current is too large during the measurement and the measurement time is too long, the resistance will change due to the heating temperature of the winding, which will increase the measurement error.

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